Effects of Sliding Conditions on Improvement of Lubrication with Micro Pits in Metal Forming

A series of experiments have been conducted using a rolling-type tribometer to investigate the mechanism and lubricating effects of the outflow behavior of lubricant trapped in a pit. Experiments using aluminum and mild steel sheets with regularly arranged pits showed that lubricant flows out along the friction stress direction and moves larger distance with increasing sliding ratio. The outflow trace becomes deeper with increasing the forming velocity or oil viscosity. Based on these facts, it was concluded that lubricant trapped in a pit flows out by MPHL (Micro Plasto-Hydrodynamic Lubrication mechanism. With an increase of sliding ratio, the friction coefficient of both the pit region and blank region increases when the blank surface is smooth, but decreases when the blank surface is rough. The lubricating improvement effects of pits are largely dependent on the lubricating regime in the blank region and disappear rapidly with the shift of the lubricating regime in the blank region from the boundary lubrication to the hydrodynamic lubrication.